Why do 360° videos look low res when uploaded to Youtube?
This has been driving me nuts too, haven't come up with a fix yet and I'm outputting using Premier Pro: unsharp mask, color correction, 4k resolution, ~30bps bitrate and 59.94 fps.  Looks great on my computer, then uploads to YouTube like garbage. 

Even 1080p/60fps renders terribly on YouTube and you would think this would be their default 'let's make this look as good as possible' configuration they'd want to get right in-terms of their compression.   

Johnpbuck submitted a video that looks decent I guess, but in my mind it's not a real world example.  Here's the quality I'm trying to get to without buying new cameras (which I don't think will solve the problem considering how good my cameras perform BEFORE YouTube) - https://www.youtube.com/watch?v=LByJ9Q6Lddo

The only trick I can suggest that others may not have tried yet is forcing rendering of your video at 59.94fps.  My cameras don't go that high but YouTube does grant a higher bitrate during uploading to a frame rate beyond 30.